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Energy question of the week: Which fuel offers the most efficient energy storage?

Whether on the road, by sea or in the air – the basis of modern transport systems is the internal combustion engine. Hardly any other invention has resulted in so many variants in just 100 years of development. One of the reasons for the technical success of diesel and petrol combustion engines is the high energy content of the fuel they burn. But how much energy do fossil fuels really contain, in comparison with hydrogen or lithium ion batteries?

Cars that consume less than three litres of fuel per 100 kilometres are often powered by diesel engines. Each litre of diesel contains 37,400 kilojoules (kJ) of energy – the equivalent of 10.4 kilowatt-hours (kWh). The next most efficient fuel after diesel is the aircraft fuel kerosene, with 36,000 kJ – 10 kWh – per litre. Petrol is a bit lower down the scale, with 34,000 kJ (9.4 kWh). A litre of fuel is lighter than water and weighs from 720 grams (petrol) to 845 grams (diesel). Biofuels made from straw, rapeseed or algae have similar energy contents. "In general, alternative, plant-based fuels could even have a higher energy content," says Marina Braun-Unkhoff from the DLR Institute of Combustion Technology in Stuttgart.

Hydrogen, which is often suggested as the fuel of the future, is a complicated affair; depending on whether it is in liquid or gas form, it can be the most efficient or the least efficient of all fuels. As a gas under normal ambient conditions (20 degrees Celsius, 1013 millibars), a litre of hydrogen has an energy content of just 10.8 kJ; in liquid form, under high pressure, it reaches 8640 kJ per litre. Since liquid hydrogen is only one-tenth as dense as petrol, liquid hydrogen can be considered to be three times as energy efficient as diesel, weight for weight. But if we look at volumes, the opposite applies and liquid hydrogen is only one-third as efficient as diesel. Since it is the energy density (energy content per volume) that determines how big a fuel tank has to be, this is a severe restriction on the design of hydrogen-powered cars – the hydrogen tank would have to be three times as large as the fuel tank of conventional car, if it is to store the same amount of energy. In addition, since the hydrogen is pressurised, the accessory equipment required for a liquid hydrogen tank is considerably more extensive than that required for the equivalent diesel or petrol tank.

Lithium ion batteries: rechargeable – but heavy

Electric cars have their own problems – the energy storage efficiency of their lithium ion batteries. A battery with a volume of one litre can store around 6500 kJ, just one-sixth of diesel's energy capacity. Lithium ion batteries are very compact, but also very heavy, so that weight for weight, they have only one-twentieth the energy density of diesel. This is why today’s electric cars have such a short range. Numerous research projects are underway around the world to increase the storage capacity of lithium ion batteries without increasing their weight.

The DLR Energy question of the week in 'The future of energy' Year of Science

The Federal Ministry of Education and Research (BMBF) has given the Year of Science 2010 the motto 'The future of energy'. For this reason the science journalist Jan Oliver Löfken will this year answer a question on the subject of energy in his blog each week. Do you have a question about how our energy supply might look in the future? Or do you want to know, for example, how a wave power plant works and how it can efficiently generate electricity? Then send us your question by email. Science journalist Jan Oliver Löfken will investigate the answers and publish them each week in this blog.

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Energy journalist Jan Oliver Löfken writes among other things, for the Technologie Review, Wissenschaft aktuell, Tagesspiegel, Berliner Zeitung and P.M. Magazin on issues involving energy research and industry. For DLR, he answered the Energy question of the week during the Year of Energy 2010.
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